Catalysts comprising a combination of platinum and palladium have been found to be useful as oxidation catalysts in catalytic converters fitted on vehicles having a diesel engine.
Another combination of metals useful in catalysis is gold and palladium. Catalysts comprising or consisting of gold and palladium on support materials are known for use as catalysts in many different chemical reactions. The use of Pd/Au catalysts in the manufacture of vinyl acetate has been well-known for many years and a large number of patents describe such processes, the catalyst and methods of making them. For example, U.S. Pat. No. 3,775,342 describes the preparation of a Pd/Au catalyst on silica for preparing vinyl esters by impregnating the support with solutions of gold and palladium compounds followed by drying and further treatment including reduction to produce palladium and gold in the metallic state. U.S. Pat. No. 5,274,181 describes, for use in vinyl acetate production, a catalyst consisting of a catalyst support having a particle diameter from about 3 to about 7 mm and a pore volume of 0.2 to 1.5 ml per gram; palladium and gold distributed in the outermost 1.0 mm thick layer of the catalyst support particles, and from about 3.5 to 9.5% by weight of potassium acetate. The catalyst is characterised by having a gold to palladium weight ratio in the range 0.60 to 1.25. Supported Pd/Au catalysts are also known for use in hydrogenation of diolefinic hydrocarbons (U.S. Pat. No. 4,533,779), hydrogenation of acetylene (WO02/078839) and oxidation (U.S. Pat. No. 6,391,821). U.S. Pat. No. 7,709,407 describes the use of supported palladium-gold catalysts for treatment of engine exhaust gases.
The catalysts of the present invention find particular utility in the treatment of exhaust gases from vehicles. However, they may also be useful in other applications.
Vehicle exhausts contain harmful pollutants including carbon monoxide, uncombusted hydrocarbons, oxides of nitrogen, soot and other particulate matter. Catalytic converters and particulate filters have been successfully used to reduce the level of pollutants emitted from vehicles. However, there is a continuing need to reduce pollution further and thus more efficient catalysts are required.
Catalytic converters used in vehicles typically contain supported particles of metal or metal compounds. The metal or metal compound particles may comprise a single metal, e.g. platinum, or they may contain two or more metals, e.g. a combination of platinum and palladium. Metals commonly used in catalytic converters include palladium, platinum, and rhodium.
The composition of vehicle exhaust gases varies depending on a large number of factors including the vehicle, the type of engine, the fuel used, how long the engine has been running, the temperature, weather conditions and others. In addition, all vehicle exhausts contain a mixture of different pollutants. In order to provide the desired level of catalytic performance it is often necessary to include a combination of different catalytic converter components, e.g. catalysed filters or three-way catalysts.